Add 'Evaluation' property to plugin.

- see
https://github.com/tuttleofx/TuttleOFX/commit/705c2b7dbad224668edfaf2194
8ab88dbccf39ae
- proposed evaluations:
  - EXR 10
  - FFmpeg 0 (doesn’t take time into account)
  - OIIO 50
  - PFM 40 (correct, but OIIO >= 1.4 also provides PFM)